“If you find that a relative or friend isn't sleeping at night, you better get someone to help them out,” Dr. Joseph Sison said.

He said lack of sleep is an early indicator of emotional illness, along with changes in appetite and frequent mood swings.

“Explosive behaviors to the extent of endangering other people or breaking things,” Sison said. “These problems don’t improve, they get worse.”

Sison said he was not surprised to learn that the shooter in the Newtown school massacre was just 20 years old.

He said the onset of a serious mental illness such as schizophrenia and bipolar disorder often occur during the teen years, or early 20s.

“I’m not surprised, and if you look a suicide rates, it spikes up at around that age,” he said. “So, it’s consistent.”

Examples of young men involved in shooting rampages include Seung Hui Cho, 23, who killed 32 people at Virginia Tech College in 2007. He suffered from severe anxiety.

Columbine High School killer Eric Harris, 18, had been diagnosed with depression and anger.

Attorneys for 24-year-old James Holmes, the suspect in the Aurora theater killings, say their client is mentally ill.

“They look like they are internally preoccupied, like possibly hearing voices and looking away and feel paranoid," Sison said. “Those are also signs to look for. And if those start coming about, then you want to get professional help.”